mx-core.mailkit.net has no CNAME records.
Question and response
Question
dig mx-core.mailkit.net. NS
Answer
The response has no answer records.
Authority
mailkit.net. 3545 SOA ns1.mailkit.net. postmaster.mailkit.net. 2026030501 28800 300 1209600 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0